1. Field
The present disclosure relates generally to a direct conversion RF receiver. More specifically, the present disclosure relates to a direct conversion RF receiver equipped with an adaptive DC offset cancellation (DCOC) circuitry.
2. Related Art
Traditional wireless communication systems are usually designed for a specific standard, such as GSM (Global System for Mobile Communications), Wideband Code Division Multiple Access (W-CDMA), Wi-Fi® (registered trademark of Wi-Fi Alliance of Austin, Tex.), LTE (Long Term Evolution), just to name a few. Current demand for the convergence of wireless services, so that users can access different standards from the same wireless device, is driving the development of multi-standard and multi-band transceivers, which are capable of transmitting/receiving radio signals in the entire wireless communication spectrum (most are in a frequency range of from 300 MHz to 3.6 GHz).
A direct-conversion receiver (DCR) directly demodulates an RF (radio frequency) modulated carrier to baseband frequencies. This is in contrast to the standard super-heterodyne receiver where an initial conversion to an intermediate frequency (IF) is needed. The simplicity of performing only a single frequency conversion reduces the basic circuit complexity, and is thus preferable for meeting the multi-band/multi-standard requirement where high-density integrated circuits (ICs) are needed.
However, one of the major challenges facing the implementation of the DCRs is the DC offset issue. In a standard DCR architecture, RF band selection is typically the only filtering performed in the receive path before the signal is down-converted directly to baseband. Therefore, a strong, nearby signal, including the receiver's own local oscillator (LO) signal, can mix itself down to zero-IF, which is known as self-mixing, and generate a DC level that appears as interference at the center of the desired band. This self-mixing is often caused by signal leakages resulting from finitely isolated substrate and bond-wire coupling. For example, in the typical silicon-based IC, port-to-port isolation (such as the isolation between the LO and RF ports of a mixer) is limited by low-resistivity substrate coupling, bond-wire radiation, and capacitive and magnetic coupling. In addition, since the LO is typically a strong signal, it can leak with sufficiently high amplitude through these unintended paths, reaching into the mixer RF input and mixing with itself, thereby generating a static DC level. The LO signal can also be radiated by the antenna, or reflected off obstructions (such as a building or a moving vehicle), and then be recaptured by the front-end and generate a DC offset. With fading and multipath reception, the received power level can vary rapidly, which results in time-varying or dynamic DC offset. In addition, DC offset can be generated by self-mixing of a strong nearby interferer or interaction of an interfering signal and circuit generated second-order nonlinearity.